Hyperglycemia is believed to be the major cause of diabetic vascular complications involving both microvessels and arteries as in the retina, renal glomeruli, and aorta. It is unclear by which mechanism hyperglycemia is altering the metabolism and functions of vascular cells, although changes in nonenzymatic protein glycosylation and increases in cellular sorbitol levels have been postulated to be involved. Previously, we have reported that the elevation of extracellular glucose levels with cultured bovine retinal capillary endothelial cells causes an increase in protein kinase C (PKC) activity of the membranous pool with a parallel decrease in the cytosol without alteration of its total activity. Now we demonstrate that the mechanism for the activation of PKC is due to an enhanced de novo synthesis of diacylglycerol as indicated by a 2-fold increase of ['4Cldiacylglycerol labeling from [14C]glucose. The elevated diacylglycerol de novo synthesis is secondarily due to increased formation of precursors derived from glucose metabolism; this formation is enhanced by hyperglycemia as substantiated by elevated [3H1glucose conversion into water. This effect of hyperglycemia on PKC is also observed in cultured aortic smooth muscle and endothelial cells and the retina and kidney of diabetic rats, but not in the brain. Since PKC in vascular cells has been shown to modulate hormone receptor turnover, neovascularization in vitro, and cell growth, we propose that this mechanism of enhancing the membranous PKC activities by hyperglycemia plays an important role in the development of diabetic vascular complications.Hyperglycemia is probably an important etiologic factor in the development of vascular complications in diabetic patients, such as retinopathy (1), nephropathy (2), and accelerated atherosclerosis (3). Nonenzymatic glycosylation (4,5) of protein and accumulation of intracellular sorbitol with reduction of myo-inositol levels (6, 7) have been proposed to be involved in the development of these vascular changes.Protein kinase C (PKC), the Ca2+/phospholipid-dependent protein kinase, appears to be involved in a variety of cellular functions such as signal transduction of cellular responses to hormones, growth factors, neurotransmitters, and drugs (8, 9). In vascular cells, PKC has been shown to modulate growth rate (10), DNA synthesis (11), hormone and growth factor receptor turnover (12), smooth muscle contraction (13-17), and cAMP responses to different hormones (18,19) and to stimulate neovascularization in vitro. Using cultured bovine retinal capillary endothelial cells, a cell type prominently involved in diabetic retinopathy (21), we have previously reported that the membranous pool of PKC activity was increased 100% by elevation of glucose level (22) but not by adding mannitol to the medium. This effect appears to be mostly an increase in the translocation of PKC from cytosol to the membrane since the total PKC activity is not altered and the maximally stimulated membranous PKC activities by phorbol 12...

Peroxidase/H2O2-mediated radical coupling of 4-hydroxycinnamaldehydes produces 8-O-4-, 8-5-, and 8-8-coupled dehydrodimers as has been documented earlier, as well as the 5-5-coupled dehydrodimer. The 8-5-dehydrodimer is however produced kinetically in its cyclic phenylcoumaran form at neutral pH. Synthetic polymers produced from mixtures of hydroxycinnamaldehydes and normal monolignols provide the next level of complexity. Spectral data from dimers, oligomers, and synthetic polymers have allowed a more substantive assignment of aldehyde components in lignins isolated from a CAD-deficient pine mutant and an antisense-CAD-downregulated transgenic tobacco. CAD-deficient pine lignin shows enhanced levels of the typical benzaldehyde and cinnamaldehyde end-groups, along with evidence for two types of 8-O-4-coupled coniferaldehyde units. The CAD-downregulated tobacco also has higher levels of hydroxycinnamaldehyde and hydroxybenzaldehyde (mainly syringaldehyde) incorporation, but the analogous two types of 8-O-4-coupled products are the dominant features. 8-8-Coupled units are also clearly evident. There is clear evidence for coupling of hydroxycinnamaldehydes to each other and then incorporation into the lignin, as well as for the incorporation of hydroxycinnamaldehyde monomers into the growing lignin polymer. Coniferaldehyde and sinapaldehyde (as well as vanillin and syringaldehyde) co-polymerize with the traditional monolignols into lignins and do so at enhanced levels when CAD-deficiency has an impact on the normal monolignol production. The implication is that, particularly in angiosperms, the aldehydes behave like the traditional monolignols and should probably be regarded as authentic lignin monomers in normal and CAD-deficient plants.

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